COLLAGEN GENE-EXPRESSION DURING DEVELOPMENT OF AVIAN SYNOVIAL JOINTS - TRANSIENT EXPRESSION OF TYPE-II AND TYPE-XI COLLAGEN GENES IN THE JOINT CAPSULE

The developmental sequence of the embryonic joint has been well studie d morphologically. There are, however, no definitive studies of cell f unction during joint development. In order to begin to understand the differentiation events that contribute to joint formation, we examined the expression of collagen mRNAs encoding types I, IIA, IIB, and XI. In situ hybridization was performed on chicken embryo hind limb buds a nd digits from day 7 to day 18 (Hamburger and Hamilton stages 31-44). In the day 7 (stage 31) limb bud, there was a condensation of mesenchy me forming the primitive tarsal and metatarsal bones that showed abund ant expression of type IIA procollagen message, but no type IIB or typ e alpha 1(XI) message. By day 8 (stage 33), co-expression of types IIA , and type XI procollagen mRNAs was observed in the condensations, wit h expression of IIB restricted to early chondrocytes with metachromati cally staining matrix. At this stage, DNA fragmentation characteristic of apoptosis was observed in cells near the midline of the interzone region between the developing anlagen, and in areas between and around the individual digits of the paddle. The presumptive apoptotic cells were more numerous at day 9 (stage 35), and were not found in the deve loping joint at subsequent time points, including the initiation of sp atial cavitation of the joint. From days 11-18, type IIA procollagen m RNA was expressed in flattened cells at the surface of the anlagen, an d in the perichondrium and in the developing joint capsule; type IIB m RNA message was found only in chondrocytes. Type XI mRNA was expressed by all type II-expressing cells. Alpha 1(I) mRNA was expressed early by cells of the interzone and capsule, but as cavitation progressed, t he type I expressing cells of the interzone merged with the superficia l layer of the articular surface. Thus, at the time of joint cavitatio n, there was a distinct pattern of expression of procollagen messages at the articular surface, with type I being outermost, followed by mor phologically similar cells expressing type IIA, then chondrocytes expr essing type IIB. The progenitor cells expressing type IIA message defi ne a new population of cells. These cell populations contribute to the molecular heterogeneity of the articular cartilage, and these same po pulations likely exist in the developing joints of other species. The transient transcription of type II and type XI collagen genes, charact eristic of chondrocytes, by cells in the joint capsule demonstrates th at these cells may have chondrogenic potential. (C) 1995 Wiley-Liss, I nc.